A reactor pressure vessel (RPV) of a boiling water reactor (BWR) typically has a generally cylindrical shape and is closed at both ends, e.g., by a bottom head and a removable top head. A top guide typically is spaced above a core plate within the RPV. A core shroud, or shroud, typically surrounds the core plate and is supported by a shroud support structure. Particularly, the shroud has a generally cylindrical shape and surrounds the both the core plate and the top guide. The top guide includes several openings, and fuel bundles are inserted through the openings and are supported by the core plate.
A plurality of openings are formed in the bottom head dome so that components, such as control rod drive assemblies, can extend within the RPV. As an example, for a control rod drive assembly, a control rod drive housing, e.g., a tube, is inserted through the bottom head dome opening and a control rod drive is inserted through the control rod drive housing. The control rod drive is coupled to a control rod to position the control rod within the core.
A nuclear reactor core includes individual fuel assemblies that have different characteristics that affect the strategy for operation of the core. For example, a nuclear reactor core has many, e.g., several hundred, individual fuel bundles that have different characteristics. Such bundles preferably are arranged within the reactor core so that the interaction between the fuel bundles satisfies all regulatory and reactor design constraints, including governmental and customer specified constraints. In addition to satisfying the design constraints, since the core loading arrangement determines the cycle energy, i.e., the amount of energy that the reactor core generates before the core needs to be refreshed with new fuel elements, the core loading arrangement preferably optimizes the core cycle energy.
In order to furnish the required energy output, the reactor core is periodically refueled with fresh fuel assemblies. To optimize core cycle energy, the higher reactivity bundles may be positioned at an inner core location. To satisfy some design constraints, however, higher reactivity bundles generally are positioned at an outer core location. The most depleted fuel bundles, i.e., the bundles with the least remaining energy content, are removed from the reactor. The interval between refuelings is referred to as a cycle of operation.
During the course of the cycle of operation, the excess reactivity, which defines the energy capability of the core, is controlled in two ways. Specifically, a burnable poison, e.g., gadolinia, is incorporated in the fresh fuel. The quantity of initial burnable poison is determined by design constraints typically set by the utility and by the NRC. The burnable poison controls most, but not all, of the excess reactivity.
Control rods also control the excess reactivity. Specifically, the reactor core contains control rods which assure safe shutdown and provide the primary mechanism for controlling the maximum power peaking factor. The total number of control rods available varies with core size and geometry, and is typically between 50 and 205. The position of the control rods, i.e., fully inserted, fully withdrawn, or somewhere between, is based on the need to control the excess reactivity and to meet other operational constraints, such as the maximum core power peaking factor.
One known control rod includes a central portion having four radially extending blades. The blades define four fuel bundle channels, and when inserting the control rod into the core, the control rod is positioned so that one fuel bundle is positioned within each channel. Therefore, for example, approximately 100 control rods are included in a reactor having 400 fuel bundles.
To reduce the number of control rods necessary for efficient operation, one known reactor includes fuel bundles with an interior water gap arranged in a K-lattice configuration. Each fuel bundle in such reactor is substantially larger than a conventional size fuel bundle, and represents twice the pitch as the conventional BWR fuel configuration. The larger fuel bundles facilitate increasing the peaking factor of the BWR core. Particularly, the maximum channel integrated power, i.e., highest radial peaking factor, is greater for such large twice pitch K-lattice fuel bundle core than for a core loaded with conventional size fuel bundles. The maximum channel peaking factor for the large twice pitch bundle core, for example, is approximately 1.7, whereas the maximum channel peaking factor for a conventional core typically is approximately about 1.4 or 1.5.
Such larger fuel bundles also facilitate reducing the number of control rod drives, and thus reduce the capital cost of the reactor. Particularly, fuel assemblies including such twice pitch bundles are approximately four times the size of conventional fuel assemblies. Accordingly, fewer twice pitch bundles may be installed in nuclear reactor as compared to standard size fuel bundles. Fewer control rods, therefore, are needed to control radiation between the fewer twice pitch bundles as compared to standard size fuel bundles. Power is generated with fewer twice pitch fuel bundles as compared to standard size fuel bundles. In addition, refueling time is decreased due to the reduced number of fuel bundles.
The twice pitch bundles provide for a nuclear reactor having a reduced number of control rod drives and a substantial reduction in capital cost as compared with a conventional reactor utilizing conventional fuel bundles. However, such larger bundles typically also require substantial redesign of the fuel assembly, (e.g., with a twice pitch bundle design, the fuel assembly is approximately four times the size of a conventional fuel assembly). Similarly, the larger bundles typically impose more parasitic material in the core, and are more susceptible to bow and bulge. In addition, the ability to perform sub-bundle shuffling, i.e., the ability to reposition individual fuel bundles within the core or remove individual fuel bundles from the core, is substantially compromised with the larger fuel bundles.
It would be desirable to reduce the number of control rod drives without requiring substantial redesign of a fuel assembly. It also would be desirable to reduce the number of control rod drives without substantially compromising the ability to perform sub-bundle shuffling.